The invention relates to a method for controlling the pressure on the cathode side of a fuel cell system.
Fuel cell systems with air supply means are disclosed for example in DE 10 2007 028 297 A1, WO2011/015282 A1 or DE 10 2008 018 863 A1. In all three cases these involve flow compressors and an exhaust air turbine disposed on the same shaft. In one case an electric motor is additionally disposed on the shaft, in the other case the turbine and flow compressor are designed as freewheels and an additional electrically driven flow compressor is disposed in series therewith.
In all three cases a so-called reorientation valve or system bypass valve is described, which connects the compressor on the output side to the exhaust air turbine or the expander on the intake side, in order then in particular to blow off compressed air again immediately when there is a danger that the compressor is approaching its surge limit.
Furthermore in fuel cell systems, in an interaction between the quantity of air conveyed through the air conveying device and a counter-pressure which is built up by suitable means in the flow direction to the cathode of the fuel cell, a corresponding control of pressure takes place, in order for example in specific situations to adapt the pressure conditions in the fuel cell. Such control elements are known in principle from the prior art. When no turbine is used, such a control device is for example a pressure-sustaining valve. In particular when, as in the above-mentioned documents, turbines are used in the exhaust air of the fuel cell system, variable turbine guide baffles can be used as pressure control means in order to establish the desired or required counter-pressure. Such variable turbine guide baffles in each case in combination with a reorientation valve are disclosed for example in WO 02/086997 A2 and DE 102 16 953 A1.
In particular a variable turbine guide baffle is extremely expensive and increases the number of components required which are susceptible to faults. Furthermore, each component used in particular in the exhaust air section is at extreme risk of freezing if the temperatures fall below the freezing point after the stopping of the fuel cell system. Furthermore, such components are expensive and require installation space, which is only available to a very small extent in particular when used by fuel cell systems in vehicles.
The object of the present invention is to provide a method for controlling pressure, in particular for increasing pressure, on the cathode side of a fuel cell system which functions without its own pressure control element.
The method according to the invention for controlling the pressure on the cathode side of a fuel cell system provides that a system bypass, which is present in any case and which connects the pressure-side outlet of the air conveying device to the pressure-side inlet of the expander, is opened in order to increase the pressure. This method according to the invention has a surprising effect in the first instance, since a volume of air conveyed via the system bypass is blown off and does not flow through the cathode side. It might be expected here that the pressure therefore drops and is not increased as required. However, the inventors have found that by the opening of the system bypass, the quantity of air which flows via the system bypass directly into the delivery-side inlet of the expander is capable of appropriately increasing the counter-pressure in the region of the expander, typically an exhaust air turbine. This leads, as for example to the case of a variable turbine guide baffle which is closed, to a higher counter-pressure which, with an almost unchanged pressure drop in the further components of the cathode side of the fuel cell system, propagates through the entire system and thus leads to the required pressure increase on the cathode side, in particular in the fuel cell or the cathode compartment thereof. In this case the desired value of the air mass flow conveyed to the fuel cell in particular remains unchanged. Thus, due to the structure it is possible at least temporarily to control pressure very simply and efficiently, and in particular without discrete pressure control elements. This is advantageous in particular if a temporary pressure increase is required, for example in order to increase the fuel cell voltage or in order to influence the humidification of the fuel cell by means of the pressure level.
For this purpose the cross-section of the system bypass through which the flow can pass is changed as a function of the required pressure. Such fine control, or a combination with a corresponding pressure measurement and a feedback message for control of the system bypass valve, even an adjustment, enables a precise adaptation of the required pressure. As usual, such a changed flow cross-section in the system bypass can of course also be achieved by means of a mere open/close valve, which is controlled by pulse width modulation so that the corresponding through-flow cross-section is established over a time average.
As already mentioned, in the method according to the invention further pressure control elements on the cathode side are omitted completely, so that the Method can be implemented very simply, cost-effectively, easily and on a small scale. Moreover, it is significantly more reliable by comparison with designs which have pressure control elements on the cathode side, since the danger of a malfunction of such pressure control elements is completely eliminated, for example because at temperatures below freezing point they are immobilized by ice.
Further advantageous embodiments of the method according to the invention are disclosed by the rest of the dependent subordinate claims and the exemplary embodiment which is described in greater detail below with reference to the drawing.